Unable to position relatively (wl_shell_surface_set_transient)

[Tarnyko]

Hi folks, 

I'm trying to use the "wl_shell_surface_set_transient" function to position 
2 surfaces relatively to each other. 

Here is a sample code, which compiles out-of-the-box with Wayland 1.2, and 
tries to map a 2nd surface at 100x100 relatively to the 1st one : 

http://pastebin.com/T0SvT1fY 

The key part is line 208 : 

wl_shell_surface_set_transient (window2->shell_surface, window->surface, 
100, 100, 0); 


But here's what I obtain : 

http://www.tarnyko.net/repo/wayland_transient_problem.png 

So the 2 surfaces keep appearing totally randomly.
What am I doing wrong ? 

Thanks for your time,
Tarnyko